JPH1149600A - Formation of layer perovskite film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the formation of the thin film having high remnant polarization and a which Curie temp. with a low treatment temp. SOLUTION: This formation comprises: a stage for preparing a first solution by dissolving bismuth 2-ethylhexanoate into a first solvent; a stage for preparing a second solution by dissolving strontium acetate into a second solvent; a stage for preparing a third solution by dissolving tantalum ethoxide into a third solvent; a stage for preparing a homogenous solution by mixing all the first to third solutions prepared in the above respective stages together; and a stage for forming a film on a substrate by depositing the homogenous solution on the substrate, wherein all the stages for preparing the above solutions are performed at the outside room temp.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates generally to nonvolatile random access memories and integrated electronic devices, and more particularly to, for example, (1-x) SrBi ₂ Ta ₂ O ₉ -xBi ₃ Ti (Ta _{1 -y} Nb respect usage of a solid solution of layered perovskite materials such as _y) O _9,
The present invention relates to a method for forming a film made of these materials using a chemical precursor solution prepared at room temperature.

[0002]

BACKGROUND OF THE INVENTION Ferroelectric thin films have received considerable attention because they can be applied to nonvolatile random access memory (NVRAM) and dynamic random access memory (DRAM) devices (JFScott et al., "
Ferroelectric Memories ", Science, 1989. The basic properties of ferroelectric materials are that they can maintain two stable remanent polarization (± P _r ) values even when the electric field is zero, that is, they are non-volatile, Is applied to cause polarization reversal from one stable polarization state to another polarization state, which makes the ferroelectric material suitable for application to a memory device. Desirable properties of the ferroelectric thin film to be applied to the present invention are that the polarization value is high, the difference between the saturation polarization (P _s ) and the remanent polarization (P _r ) is small, and the coercive electric field (E _c ) is small. And most importantly, high durability to withstand multiple inversions.In addition, to be useful, ferroelectric materials used in memory arrays require high retention and imprint properties. Must have This film integrated circuit (I
It must have a very uniform composition and thickness over the entire surface of C). Thereby, the capacitance associated with each memory cell becomes equal. Also, the processes required to form the ferroelectric thin film must not have a detrimental effect on the circuits located below the thin film.

Many materials exhibit ferroelectricity, but two material systems, namely perovskites (eg, PbZr _1-x Ti _x O ₃ )
And layered perovskites (eg SrBi ₂ Ta ₂ O ₉ and SrBi ₂ Nb
₂ O ₉ ) and research on its application to memory has been widely conducted. Important parameters for application to memory are polarization reversal, fatigue, polarization retention, imprint and leakage current characteristics.

The realization of commercially feasible, non-volatile FRAM technology based on PZT is an issue of the reliability of PZT ferroelectric capacitors, or the problems associated with the growth and processing of ferroelectric capacitor layers. It has been hampered by one of them, or a combination of those challenges. P
PZT films grown on metal electrodes such as t show high fatigue, ie, a decrease in polarization with switching cycles. The problem with polarization fatigue is that, for practical purposes, instead of a metal Pt electrode, a metal oxide electrode such as RuO ₂ or any perovskite metal oxide electrode such as La _0.5 Sr _0.5 CoO ₉ Or only by using a hybrid metal oxide electrode. Although lead-based ferroelectric materials have been widely studied, due to problems in fatigue, environmental safety and health concerns, recently, SrBi ₂ Ta ₂ O ₉ (a member of the layered perovskite system) Interest in SBT) has grown. Layered perovskite materials are attractive because of their excellent fatigue, retention, and electrical properties. SBT has low leakage current, good fatigue and retention properties,
It is a promising material for memory applications. The main limitations in using SBT materials are the high processing temperatures (800-850 ° C.), the relatively low remanent polarization Pr and the low remanent polarization that make it very difficult to integrate directly into high density CMOS devices. Curie temperature is required. By comparing PZT and SBT based capacitors (advantages and disadvantages)
The main points are shown below.

The PZT-based capacitor is a polycrystalline SBT-based capacitor (up to 20 μm) which has been developed so far.
C / cm ²⁾ with a larger polarization (40-50μC / cm ^2).

A PZT layer having a pure perovskite structure and good electrical properties generally has a temperature (600-850 ° C.) lower than the formation temperature (750-850 ° C.) of the SBT layer depending on the specific film deposition technique. 700 ° C).

PZT-based capacitors require oxide or hybrid metal oxide electrode technology to achieve the two important electrical properties of FRAM, negligible fatigue and imprint. The synthesis of these electrodes is more complicated than the synthesis of pure metal electrodes such as Pt.

[0008] The PZT-based capacitor contains Pb. Pb has caused pollution problems and harmful problems in the manufacturing process.

[0009] SBT-based capacitors exhibit negligible fatigue and imprint using simpler Pt electrode technology.

The SBT layer has good electrical properties, even when very thin (<100 nm).

The main problems to be overcome to realize a practical high-density memory device are described below.

SBT-based capacitors have a lower polarization than PZT-based capacitors. When designing capacitors to the submicron dimensions required for these memories, intrinsically high polarization values may be essential. this is,
This can be a problem in applying SBT to a high-density memory.

S having an appropriate layered perovskite structure
BT synthesis depends on the high temperature (800-850) depending on the film deposition technology.
° C). This processing temperature is higher than the processing temperature of standard semiconductor devices, so that efforts to reduce the temperature are required.

The Curie temperature of SBT is low (~ 310
° C). Since ferroelectric properties strongly depend on Curie temperature,
Higher Curie temperatures are required for memory applications. As the operating temperature approaches the Curie temperature, the ferroelectric polarization decreases rapidly. Thus, to maintain a stable and constant polarization, the Curie temperature needs to be significantly higher than the operating temperature range.

[0015]

[Problem to be Solved by the Invention] Paper "A Family of Ferroelectric Bismuth Compounds" by ECSubbarao
(J. Phys. Chem. Solids, Vol. 23, pp. 665-676 (196
2)) describes a material having a composition of SrBi ₂ N ₆₂ O ₉ —Bi ₃ TiN ₆ O ₉ . The above article describes bulk materials and does not describe properties or applications. Also,
The article does not describe or suggest a compound by a film-forming method as described herein below.
U.S. Patent No. 5,423,285 (Araujo et al., 1993) describes certain aspects in the production of layered perovskite materials. However, it does not describe or suggest the formation or utilization of a solid solution, as described herein below. The compounds described in Araujo et al. Have lower ferroelectric properties at 750 ° C. annealing temperature than the compounds of the present invention. Moreover, the present invention provides relatively high ferroelectric properties at annealing temperatures below 650 ° C. In addition, according to the forming method of the present invention, a precursor solution can be prepared at an ambient temperature (such as lower than about 35 ° C.). This is itself a remarkable advance from the prior art. This is because the corresponding manufacturing methods known to the applicant of the prior patent (such as Araujo et al.) Require that the precursor solution be heated at an elevated temperature, such as above 70 ° C.

The ultimate quality of a ferroelectric thin film depends on the intrinsic properties of the material, processing techniques, annealing, and extrinsic factors such as substrates and electrodes. S
BT is currently the most promising material. However, in the technology related to SBT-based capacitors, high processing temperatures,
The challenges of low remanent polarization and low Curie temperature must be overcome.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a method for forming a thin film having a high remanent polarization and a high Curie temperature at a low processing temperature. .

[0018]

The method for forming a layered perovskite film according to the present invention comprises the steps of dissolving bismuth 2-ethylhexanoate in a first solvent to prepare a first solution; and forming strontium acetate in a second solution. A step of preparing a second solution by dissolving in a solvent, a step of preparing a third solution by dissolving tantalum ethoxide in a third solvent, and mixing the solutions prepared in each step to form one solution. A step of preparing a homogeneous solution, and a step of depositing the homogeneous solution on a substrate to form a film on the substrate. Is achieved.

Preferably, the method further includes a step of annealing the substrate and the thin film at a temperature between 600 ° C. and 750 ° C. Further, the first solvent may be 2-ethylhexanoic acid, the second solvent may be acetic acid, and the third solvent may be 2-methoxyethanol. Also, about 30% excess bismuth may be added. Further, the method may include a step of forming an upper electrode and a lower electrode such that the two electrodes sandwich the film to form a capacitor structure.

The method for forming a layered perovskite film according to the present invention comprises the steps of dissolving bismuth 2-ethylhexanoate in a first solvent to prepare a first solution, and dissolving strontium acetate in a second solvent. A step of preparing a second solution, a step of dissolving isopropyl titanate and tantalum ethoxide in a third solvent to prepare a third solution, and mixing the solutions prepared in each step to form one Including a step of preparing a homogeneous solution and a step of depositing the homogeneous solution on a substrate to form a (1-x) SrBi ₂ Ta ₂ O ₉ -xBi ₃ TiTaO ₉ film on the substrate The above object is achieved by performing these steps at ambient room temperature.

Preferably, the method further includes a step of annealing the substrate and the thin film at a temperature between 600 ° C. and 750 ° C. Further, the first solvent may be 2-ethylhexanoic acid, the second solvent may be acetic acid, and the third solvent may be 2-methoxyethanol. Further, the step of forming an upper electrode and a lower electrode may further include a step of forming a capacitor structure by sandwiching the film between the two electrodes.

The method for forming a layered perovskite film of the present invention comprises the steps of dissolving bismuth 2-ethylhexanoate in a first solvent to prepare a first solution, and dissolving strontium acetate in a second solvent. Preparing a third solution by dissolving isopropyl titanate, tantalum ethoxide, and niobium ethoxide in a third solvent; and mixing the solutions prepared in each step. Preparing a homogenous solution by heating and depositing the homogenous solution on a substrate, and depositing (1-x) SrBi ₂ Ta ₂ O ₉ -xBi ₃ on the substrate.
And a step of forming a TiNbO ₉ film. The above object is achieved by performing these steps at an ambient room temperature.

It is preferable that the method further includes a step of annealing the substrate and the thin film at a certain temperature between 600 ° C. and 750 ° C. Further, the first solvent is 2
-Ethylhexanoic acid, the second solvent may be acetic acid, and the third solvent may be 2-methoxyethanol. Also, in the step of forming an upper electrode and a lower electrode, the two electrodes sandwich the film,
A step of forming a capacitor structure may be included.

The operation will be described below. According to the present invention,
The use of an alkoxide salt precursor solution allows for mixing of the precursor solution at room temperature and allows for processing of the finished device at lower temperatures. Furthermore, SB
T, (1-x) SrBi 2 Ta 2 O 9 -xBi 3 TiTaO 9 and _{(1-x) SrBi 2 Ta} 2 O 9 -
xBi ₃ TiNbO ₉ can be produced at room temperature.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The approaches, considerations and processes in the selection of a proposed new solid solution material, and a modified chemical preparation technique to overcome the problems with SBT-based capacitor technology, are described below.

In order for the SBT layer to have an appropriate layered perovskite structure, a high temperature (800-850 ° C.) depending on the film deposition technique is required. At lower temperatures, the microstructure and ferroelectric properties of the film are not good. This phenomenon can occur due to the polycrystalline nature of the SBT layer related to the polarization direction of the SBT material. When designing capacitors to the submicron dimensions required for these memories, intrinsically high polarization values may be essential. The idea of using solid solution materials to solve these issues is based on a bulk ceramic approach. The bulk ceramic approach modifies material properties by creating solid solutions of different materials that have different microstructures and Curie temperature properties, but similar structures (see ECSubbarao, supra). The Curie temperature of SBT is 310 ° C., whereas Bi ₃ TiTa _1-y Nby _y O ₉ has a Curie temperature in the range of 870-950 ° C. Both of these materials belong to the layered perovskite system. Thus, it is believed that a solid solution of these two materials exhibits a higher Curie temperature.

The saturation polarization value (Ps) of the Bi ₃ TiNbO ₉ material is 2
7.7 μC / cm ² , whereas bulk S
The saturation polarization value (Ps) of the BT material is reported to be 5.8 μC / cm ² . Therefore, the solid solution of these two materials is SBT
It is considered to show a higher polarization value as compared with.

At lower annealing temperatures, the ferroelectric properties are poor, so the post-deposition annealing temperature cannot be lowered. This problem is due to the intrinsic nature of the material, which is due to the smaller particle size at lower annealing temperatures. The Bi ₃ TiTa _1-y Nby _y O ₉ material shows a significantly larger particle structure compared to SBT under similar annealing conditions. Therefore, these two
The solid solution of the two materials, at lower annealing temperatures,
It is believed that a larger particle size is formed as compared to BT. Therefore, this solid solution solves the problem of lowering the processing temperature.

Similar ideas can be applied to the preparation of other solid solution materials. Solid solutions of layered perovskite materials can be made between materials that can be classified into three general types (GASmolenskii):
et al., "Ferroelectrics and related materials", Go
rdon and Breach Science Publishers, New York, 1984
Reference): 1. A compound represented by the chemical formula A _m-1 Bi ₂ M _m O _{3m + 3} . Where A =
Bi ³⁺ , Ba ²⁺ , Sr ²⁺ , Ca ²⁺ , Pb ²⁺ , K ⁺ , Na ⁺ and other ions of similar size, M = Ti ⁴⁺ , Nb ⁵⁺ , Ta ⁵⁺ , Mo ⁶⁺ , W ⁶⁺ , Fe ³⁺ and 8
Other ions that can occupy the oxygen level of the facepiece. this is,
Include compounds such as the Bi ₄ Ti ₃ O ₁₂ and SrBi ₂ Ta ₂ O _9.

2. A compound represented by the chemical formula A _{m + 1} M _m O _{3m + 1} . This is because strontium titanate Sr ₂ TiO ₄ , S
include compounds such as r ₃ Ti ₂ O ₇ and Sr ₄ Ti ₃ O _10.

3. A compound represented by the chemical formula A _m M _m O _{3m + 2} .
This includes compounds such as Sr ₂ Nb ₂ O ₇ and La ₂ Ti ₂ O ₇ .

These layered perovskite materials are metal oxide complexes which spontaneously form a layered perovskite structure.
(complex oside of metals) (This metal is strontium, calcium, barium, bismuth, cadmium,
Lead, titanium, tantalum, hafnium, tungsten, niobium, zirconium, scandium, yttrium, lanthanum, antimony, chromium and thallium.
Contains. Generally, each layered perovskite material contains more than one of the above metals. If the material contains elements that evaporate or otherwise disappear during processing, such as lead, bismuth, thallium, and / or antimony, excess content in the range of 1-100% should be optimized. Characteristics can be obtained. It is also possible to dope these solid solution materials in order to modify the characteristics according to the use of various electronic devices. The present invention
A simple room temperature chemical precursor solution preparation technique for forming a pyrochlore-free crystal film at a low annealing temperature is provided. A chemical precursor solution approach using a combination of carboxylate and alkoxide has been developed that forms a pyrochlore-free crystalline phase even at low annealing temperatures. The main features of the process are its preparation at room temperature, short preparation time, availability of precursors, stability and compatibility with semiconductor manufacturing technology. Longer manufacturing processes are more costly in terms of equipment and labor, so shorter processes are desirable. This process is applicable to the production of perovskites, pyrochlores, layered perovskites, or tungsten bronze materials and their solid solutions. The same approach is also useful for doping these materials.

This process is simple and compatible with conventional integrated circuit materials and processes. In this process, first, a precursor solution is prepared containing the respective metals of the desired thin film compound. The method for preparing a precursor solution for forming an oxide thin film includes (1) a method using only an alkoxide, (2) a method using an alkoxide salt, (3) other methods,
There are three general methods. These techniques form an oxide network through hydrolysis and condensation of precursor molecules. This chemical reaction is based on the rate of hydrolysis,
Controlled by factors such as catalysis and the molecular structure of the precursor. In the latter, nucleophilic chemical additives that modulate the coordination shell of the metal, such as organic acids (β-diketones or allied derivatives), are oligomerized, solvated or added. By doing
It can be easily modified. The selection of the precursor compound and its solvent is the most important step in the approach using a precursor solution for forming a thin film. The most important points to consider are the nature of the initial material type, the mixed metal type (m
ixed-metal species), whether the stoichiometry of the various elements in solution is appropriate for the required chemical formula, the action of the solvent, the reaction temperature and its effect on homogeneity at the molecular level, Removal of organic substances from the deposited thin film and crystallization temperature. In the method using only an alkoxide, as a first step, an alkoxide containing each element of a desired compound is selected, and one solution containing a necessary metal cation is synthesized. Generally, the solution is a metal alkoxide [M (OR) _x ] dissolved in an alcohol solvent.
Consists of When water is added to the alkoxide solution, hydrolysis occurs, followed by condensation reaction and formation of a network structure, and finally, the polymer gel has a continuous structure. Many types of alkoxides have been synthesized and used for thin film formation, but some metal alkoxides have low solubility, are difficult to prepare, and are not stable over time. Most metal alkoxides are sensitive to hydrolysis and condensation. The exclusive use of alkoxides for multi-component solutions is sometimes not possible and requires alternative precursors. Most improved ceramics are multi-component materials that have more than one type of cation in the lattice. Since the alkoxide precursor is mixed with the solution at the molecular level, high homogeneity is required. However, a major challenge in preparing a homogeneous multi-component solution is that the rates of hydrolysis and condensation of the respective metal alkoxides are different. As a result, phase separation may occur during hydrolysis or heat treatment. Phase separation raises the crystallization temperature and produces unwanted crystalline phases. Therefore, it is necessary to prepare a highly homogenous solution in which different types of cations are homogeneously distributed on the atomic scale between the MOM 'bridges in the solution. Thus, prior to gelation, the initial solution must first be treated so that binding occurs between the various alkoxide precursors. The difficulty with the alkoxide-only approach, especially with respect to soluble alkoxides, is that mixed metal species (mixed-medium
tal species) and determine the stoichiometry between the metals. The solubility of a metal alkoxide in the presence of another metal alkoxide is a criterion that requires careful handling because it does not necessarily mean formation of mixed metal species, that is, homogeneity at the molecular level. For example, despite the preparation of a homogeneous solution, no reaction occurs between bismuth 2-methoxyethoxide and titanium 2-methoxyethoxide (IR and ¹ H NMR verification). In a compound containing bismuth as one element, selection of a precursor is problematic due to lack of reactivity of bismuth alkoxide. Bismuth alkoxide, Bi (OR) ₃ R = Et, iP
It is known that r, does not react with various metal alkoxides including niobium, tantalum, titanium and lead.

According to the approach using an alkoxide salt, many of these problems of the alkoxide-only method can be overcome. The term salt in the processes described basically refers to carboxylate salts, but also includes but is not limited to nitrates, sulfates, carbonates, chlorides and hydroxides. Alkoxides and carboxylate salts fall into the group of organic derivatives of metals having a metal-oxygen-carbon bond. For some elements of the desired compound, salts can be selected as precursors instead of alkoxides, so using the alkoxide salt approach overcomes many of the challenges of the alkoxide-only approach Can be. For most metals, metal carboxylate with a medium length ligand is the preferred precursor compound.
Carboxylate anions (general formula RCOO ^-) is the variability of the ligand (1) merely exists as a counter anion, or (2) a metal (a) a monodentate, (b) a chelating or
(c) It is possible to combine them by a bridging method. A wide variety of available groups (eg, R can be H, alkyl, allyl, perfluoroalkyl, etc.)
Greatly enhances the variability of carboxylate ligands.
Metal carboxylates have the important feature of forming bonds between metals. Metal acetate or 2-ethylhexanoic acid precursor acts on most metals. They act as active metal oxide sources and participate in reactions with metal alkoxides. Acetates of divalent and trivalent metals are highly reactive towards metal alkoxides and are often easily incorporated into mixed metal species under mild conditions. For example, acetates of magnesium, cadmium, barium, strontium, and lead are separated and incorporated in mixed metal species at room temperature. The choice of solvent is also important in approaches using alkoxide salts. Solvents can have a variety of effects. The solvent forms and / or stabilizes the intermediates, so that the reaction can be prepared or modified. The presence of alcohol
Helps form reactive spiecies. Alkoxides - The main advantage of the approach of using a carboxylate, a by-product of the reaction is that it is relatively unreactive compared with H ₂ O, a by-product approach using only alkoxides. In all alkoxide acetates, the carboxylate ligand is in the bridge position and connects the different metals. In general, alkoxide acetates are more soluble than simple alkoxides, especially in parent alcohols. Most metal alkoxides are very sensitive to hydrolysis and condensation. These must be stabilized to avoid settling. These reactions are controlled by the addition of complexing agents that react with metal alkoxides at the molecular level, producing precursors with new molecules having different structures, reactivity and functionality. Carboxylic acids such as acetic acid and β-diketones (primarily acetylacetone) act as hydroxylated nucleophilic ligands, reducing the rate of hydrolysis of precursors by reducing their functionality. Help control. The alkoxide and carboxylate in the presence of the alcohol and carboxylic acid react to form the smallest possible aggregates. This aggregation allows the metal to obtain the most common coordination numbers, thus making hydrolysis more difficult. Carboxylates have the property of increasing the nucleophilicity of aggregates because they act as ligands to supply assembly and oxo, while diketones are chelating ligands, which can lead to oligomerization. Has the property of reducing. FIG.
The general steps of the film forming method according to the present invention are shown below. Step a, which is the first step, is the selection of a precursor compound and a solvent. In forming a thin film by a chemical method using a precursor solution, selection of a precursor compound and a solvent is an important step.
The shelf life of the precursor for each metal must be long. The selected precursor must have high solubility in the selected solvent. Also, various solvents must be compatible when mixed. The resulting precursor solution has a relatively long shelf life, so that it can be prepared in large quantities in advance and used as needed. Each step in the alkoxide salt approach is shown below.

1. Selection of metal alkoxide or carboxylate as initial precursor.

The precursor must be a relatively small group of organics that form a ligand with a metal that eventually becomes a thin film contained in the material. This minimized the amount of organic material that had to evaporate, thus minimizing the size of pores and other fine defects in the film. In addition, short-chain metal carboxylate usually has very high polarity, and thus has high water solubility. Therefore,
It does not separate when hydrolyzed water is added to the solution. However, it has the property of being insoluble in high-boiling solutions such as xylene and 2-methoxyethanol if the polarity is high. On the other hand, all long-chain metal carboxylates, such as neodecanoate and 2-ethylhexanoic acid, are generally soluble in either 2-methoxyethanol or xylene. However, they are insoluble in water. Thus, if a substantial amount of metal neodecanoate or metal 2-ethylhexanate is present, the hydrolyzed water and the alkoxide gel separate around the water droplets formed when the hydrolysis is performed. . In addition, a long-chain material contains a large amount of organic material, so that an excellent film cannot be formed. Thus, metal carboxylate having a medium length ligand (with about 10 or less carbon atoms attached in the chain) is more suitable for preparing a precursor solution having a long shelf life and forming a high quality thin film. I have. Preferably, the metal carboxylate is a metal carboxylate having a medium chain ligand, such as metal acetate or metal 2-ethylhexanate. These are applicable to most metals. Alternatively, nitrates, lead sulfates, carbonates, chlorides and hydroxides can be selected as precursors.

2. A carboxylic acid can be selected as a solvent for the metal carboxylate. Solvent boiling point 100 ℃
Desirably higher, in the range of 100-250 ° C.
Suitable solvents for the alkoxide salt precursor solution are alcohols, aromatic hydrocarbons, ketones, esters, ethers and alkanolamines. One solvent or a combination of multiple solvents can be used to optimize solubility and viscosity to obtain high quality coatings.

In step b, each precursor is dissolved in a selected solvent, and then these solutions are mixed to finally obtain a homogeneous solution. In step c, the final solution is subjected to hydrolysis and polycondensation to stabilize the final solution.
This is usually treated by carboxylic acid, beta diketone, alcoholysis and / or hydrolysis. In step d, a precursor solution is deposited on the substrate. Films can be formed from precursor solutions using spin, dipping, or spray techniques. The alcohol solution can generally soak any metal, oxide, or semiconductor substrate that has a thin oxide layer on the surface. Almost any substrate that supports a thin film, is compatible with the film material, and can withstand processing can be used. Here, the thin film was formed by a spin coating technique. The spin coating technique is a well-known deposition method in which a precursor is placed on a wafer, and the wafer is rotated to uniformly distribute the precursor over the entire surface of the wafer. In order to control the film thickness, it is necessary to optimize the spin speed and the viscosity of the solution.
Preferably, the wafer is rotated at a spin speed in the range of 1000 rpm to 7000 rpm for 5 seconds to 120 seconds. The film formed on the substrate in step d is a wet film. In step e, the film is baked to remove organic contents. After the coating process (step d), the wafer is transferred onto a hot plate where it is baked. Alternatively, if control of ambient conditions is desired, baking may be performed using an oven. Preferably, the baking is performed in a temperature range of 150-350 ° C for a time between 1-15 minutes. The drying step may be arbitrarily set between the coating and baking steps. Preferably, drying is in a temperature range
Perform at 150-200 ° C. If the desired film thickness cannot be obtained by one coating, spin,
The steps of drying and baking, that is, steps d and e are repeated. In step f, after the final layer has been coated and baked, the film is annealed in a diffusion furnace or rapid thermal annealing system. Annealing is preferably performed in various flow rates of oxygen atmosphere for various times and at various temperatures. Crystal films having different morphologies can be obtained depending on the type of substrate and the processing method. The most common products are polycrystalline films without specific crystallographic orientation. The polycrystalline film is usually obtained by using any type of substrate among a polycrystalline substrate, an amorphous substrate, and a single crystal substrate having a large lattice mismatch. Texture oriented films can be obtained under some special conditions. First, when a single crystal substrate having a considerably large lattice mismatch is used, a film grown on the entire surface preferably has a high degree of orientation. Alternatively, applying a small dc (direct current) bias electric field along the substrate surface during the post-deposition annealing process can grow a film with suitable orientation. Films annealed by the rapid thermal annealing process also exhibit suitable orientation, depending on the conditions. When a single crystal substrate having small lattice mismatch is used, an epitaxial film can be formed in a ferroelectric system.

(Embodiment 1) As described above, Sr is obtained by a chemical precursor solution method using a solution preparation method at room temperature.
A Bi ₂ Ta ₂ O ₉ film was formed. To make SrBi ₂ Ta ₂ O ₉ ,
Bismuth 2-ethylhexanoate, strontium acetate and tantalum ethoxide were selected as precursors, and acetic acid, 2-ethylhexanoic acid and 2-methoxyethanol were selected as solvents. The selected precursors had high solubility in corresponding solvents under ambient room temperature conditions. 2
Bismuth-ethylhexanoate was dissolved in 2-ethylhexanoic acid, strontium acetate was dissolved in acetic acid, and tantalum ethoxide solution was formed in 2-methoxyethanol solution. Bismuth was added in excess, taking into account the loss of bismuth during processing. The best results were obtained when a 30% excess of bismuth was added. Thereafter, the various solutions were mixed. The final solution was stable and clear.
The viscosity and surface tension of the solution were controlled by changing the content of 2-methoxyethanol. In the present embodiment, the annealing of the film is performed in an oxygen atmosphere in a temperature range of 600-
Performed at 750 ° C. FIG. 2 shows an X-ray diffraction pattern of the film of the present embodiment. This membrane has a pyrochlore secondary phase (s
It did not contain any econdary phase, and was found to be well crystallized at 650 ° C. It was found that the intensity and sharpness of the peak of the X-ray diffraction pattern increased as the annealing temperature increased. This indicates an increase in crystal grain size and crystallinity. FIG. 3 is an atomic force micrograph of the film annealed at 750 ° C. This film shows a dense microstructure without cracks and defects.
Electrical properties were obtained for films in the metal-ferroelectric film-metal (MFM) configuration. An MFM capacitor was formed by depositing a platinum electrode through a mask on the upper surface of the film by sputtering. The lower platinum electrode was placed by etching the film. The dielectric constant of the film annealed at 750 ° C is 100 kHz
It was 330 at the frequency of z. FIG. 4 is a diagram showing a PE hysteresis loop in a film annealed at 750 ° C. The remanent polarization value (2Pr) of the SrBi ₂ Ta ₂ O ₉ thin film is 17.2
μC / cm ² and the coercive electric field value were 23 kV / cm. As shown in FIG. 5, this film exhibited good switching resistance under switching cycles with at least 10 ¹⁰ bipolar loads.

(Embodiment 2) (1-x) SrBi ₂ Ta ₂ O ₉ -xBi ₃ TiTa
In forming the O ₉ thin film, bismuth 2-ethylhexanoate, strontium acetate, isopropyl titanate and tantalum ethoxide were selected as precursors, and acetic acid,
-Ethylhexanoic acid and 2-methoxyethanol were selected as solvents. The selected precursors had high solubility in the corresponding solvents at ambient room temperature. Bismuth 2-ethylhexanoate was dissolved in 2-ethylhexanoic acid, strontium acetate was dissolved in acetic acid, and a solution containing tantalum ethoxide and isopropyl titanate was formed in a 2-methoxyethanol solution. Bismuth was added in excess, taking into account the loss of bismuth during processing. Thereafter, the various solutions were mixed. The final solution was stable and clear. The viscosity and surface tension of the solution were controlled by changing the content of 2-methoxyethanol. In this embodiment, the film of the compound 0.7SrBi ₂ Ta ₂ O ₉ -0.3Bi ₃ TiTaO ₉ was annealed in an oxygen atmosphere at a temperature range of 600 to 750 ° C. FIG. 6 shows an X-ray diffraction pattern of the film of the present embodiment. This film contained neither pyrochlore nor secondary phases, and was found to be well crystallized at 600 ° C.
It was found that the intensity and sharpness of the peak of the X-ray diffraction pattern increased as the annealing temperature increased. This indicates an increase in crystal grain size and crystallinity. FIG. 7 is an atomic force micrograph of the film annealed at 750 ° C. This film shows a dense microstructure without cracks and defects. It was found that the grain size was significantly improved compared to the SrBi ₂ Ta ₂ O ₉ film annealed under similar conditions (FIG. 3). Electrical properties were obtained for films in the metal-ferroelectric film-metal (MFM) configuration. By depositing a platinum electrode through a mask on the upper surface of the film by sputtering, M
An FM capacitor was manufactured. The lower platinum electrode was placed by etching the film. The dielectric constant of the film annealed at 750 ° C. is 200 at a frequency of 100 kHz.
Met. FIG. 8 is a diagram showing a PE hysteresis loop in a film annealed at 750 ° C. 0.7SrB
The remanent polarization (2Pr) value of the i ₂ Ta ₂ O ₉ -0.3Bi ₃ TiTaO ₉ thin film is 2
The coercive electric field value was 7.8 μC / cm ² and 68 kV / cm. Also, 650
The film of the present embodiment that has been annealed at C ° C. is SrBi ₂ Ta ₂ O ₉
It also showed significantly improved ferroelectric properties compared to the films (Table 1). Low temperature processing is desired to help select a suitable barrier layer for the memory device. It can be seen that the 2P _r values were significantly improved for solid solutions (Table 1). As shown in FIG. 9, the film exhibited good switching resistance under switching cycles with at least 10 ¹⁰ bipolar loads.

(Embodiment 3) (1-x) SrBi ₂ Ta ₂ O ₉ -xBi ₃ TiNb
O ₉ in the formation of thin films, bismuth 2-ethylhexanoate, strontium acetate, isopropyl titanate, tantalum ethoxide and niobium ethoxide were selected as precursors, acetate, 2-ethylhexanoate and 2-methoxyethanol as a solvent Selected. The selected precursors had high solubility in the corresponding solvents at ambient room temperature. Bismuth 2-ethylhexanoate dissolves in 2-ethylhexanoic acid, strontium acetate dissolves in acetic acid,
A solution containing niobium ethoxide, tantalum ethoxide and isopropyl titanate was formed in a 2-methoxyethanol solution. Bismuth was added in excess, taking into account the loss of bismuth during processing. Thereafter, the various solutions were mixed. The final solution was stable and clear.
The viscosity and surface tension of the solution were controlled by changing the content of 2-methoxyethanol. In this embodiment, the film of the compound 0.8SrBi ₂ Ta ₂ O ₉ -0.2Bi ₃ TiNbO ₉ was annealed in an oxygen atmosphere in a temperature range of 600 to 750 ° C.
FIG. 10 shows an X-ray diffraction pattern of the film of this embodiment.
This membrane contains neither pyrochlore nor secondary phases,
It was found that crystallization was good at 600 ° C. It was found that the intensity and sharpness of the peak of the X-ray diffraction pattern increased as the annealing temperature increased. This indicates an increase in crystal grain size and crystallinity. FIG. 11 is an atomic force micrograph of the film annealed at 750 ° C. This film shows a dense microstructure without cracks and defects. It was found that the grain size was significantly improved compared to the SrBi ₂ Ta ₂ O ₉ film annealed under similar conditions (FIG. 3). Electrical properties were obtained for films in the metal-ferroelectric film-metal (MFM) configuration. By depositing a platinum electrode through a mask on the upper surface of the film by sputtering, M
An FM capacitor was manufactured. The lower platinum electrode was placed by etching the film. The dielectric constant of the film annealed at 750 ° C. is 200 at a frequency of 100 kHz.
Met. FIG. 12 is a diagram showing a PE hysteresis loop in a film annealed at 750 ° C. 0.8S
The remanent polarization value (2Pr) of the rBi ₂ Ta ₂ O ₉ -0.2Bi ₃ TiNbO ₉ thin film is 2
The coercive electric field value was 6.9 μC / cm ² and 68 kV / cm. Also, 650
The film of the present embodiment that has been annealed at C ° C. is SrBi ₂ Ta ₂ O ₉
It also showed significantly improved ferroelectric properties compared to the films (Table 1). The 2Pr value was found to be greatly improved in the solid solution (Table 1). As shown in FIG. 13, this film exhibited good switching resistance under switching cycles with at least 10 ¹⁰ bipolar loads applied. The following table shows the experimental results of the first to third embodiments. This indicates that the ferroelectric characteristics have been improved.

[0042]

[Table 1]

[0043]

According to the present invention, it is possible to provide a method for producing a solid solution of a layered perovskite material, which is used particularly as a thin film for a nonvolatile random access memory and an integrated electronic device. In the present invention, the use of the alkoxide salt precursor solution allows mixing of the precursor solution at room temperature and also allows processing of the completed device at lower temperatures. Furthermore, SBT, (1-x) S
rBi ₂ Ta ₂ O ₉ -xBi ₃ TiTaO ₉ and (1-x) SrBi ₂ Ta ₂ O ₉ -xB i ₃ TiNbO
₉ at room temperature. The present invention provides a room temperature chemical precursor solution preparation technique that has been developed to form pyrochlore-free, high quality crystalline films at post-deposition annealing temperatures as low as 650 ° C.

[Brief description of the drawings]

FIG. 1 is a diagram showing a flow diagram in the production of a layered perovskite material and a solid solution thereof by a chemical precursor solution method using an alkoxide salt precursor solution prepared at room temperature.

FIG. 2 shows the XRD pattern of a SrBi ₂ Ta ₂ O ₉ thin film as a function of annealing temperature.

FIG. 3 is an atomic force micrograph of a SrBi ₂ Ta ₂ O ₉ thin film annealed at 750 ° C.

FIG. 4 is a diagram showing a typical PE hysteresis loop in a SrBi ₂ Ta ₂ O ₉ thin film.

FIG. 5 shows the decay of polarization in a SrBi ₂ Ta ₂ O ₉ thin film as a function of the number of switching cycles.

FIG. 6 shows the XRD pattern of 0.7SrBi ₂ Ta ₂ O ₉ -0.3Bi ₃ TiTaO ₉ thin film as a function of annealing temperature.

FIG. 7: 0.7SrBi ₂ Ta ₂ O ₉ -0.3 annealed at 750 ° C.
4 is an atomic force microscope photograph of a Bi ₃ TiTaO ₉ thin film.

FIG. 8 is a diagram showing a typical PE hysteresis loop in a 0.7SrBi ₂ Ta ₂ O ₉ -0.3Bi ₃ TiTaO ₉ thin film.

FIG. 9 shows the decay of polarization in a 0.7SrBi ₂ Ta ₂ O ₉ -0.3Bi ₃ TiTaO ₉ thin film as a function of the number of switching cycles.

FIG. 10 shows the XRD pattern of 0.8SrBi ₂ Ta ₂ O ₉ -0.2Bi ₃ TiTaO ₉ thin film as a function of annealing temperature.

FIG. 11: 0.8SrBi ₂ Ta ₂ O ₉ -0.2Bi annealed at 750 ° C.
_{3 is} an atomic force micrograph of a 3TiTaO ₉ thin film.

FIG. 12 is a diagram showing a typical PE hysteresis loop in a 0.8SrBi ₂ Ta ₂ O ₉ -0.2Bi ₃ TiTaO ₉ thin film.

FIG. 13 shows the decay of polarization in a 0.8SrBi ₂ Ta ₂ O ₉ -0.2Bi ₃ TiTaO ₉ thin film as a function of the number of switching cycles.

────────────────────────────────────────────────── (5) Continuation of the front page (51) Int.Cl. ⁶ Identification symbol FI H01L 21/8242 H01L 29/78 371 21/8247 29/788 29/792 (72) Inventor Seshby. Death United States of America Virginia 24061-0237, Blacksburg, Virginia Tech, Holden Hall 213 (72) Inventor Pulancy. Josie United States of America Virginia 24061-0237, Blacksburg, Virginia Tech, Holden Hall 213 (72) Inventor Schweigian United States of America Virginia 24061-0237, Blacksburg, Virginia Tech, Holden Hall 213 (72) Inventor Son Oh. Liu United States Virginia 24061-0237, Blacksburg, Virginia Tech, Holden Hall 213

Claims (13)

[Claims] A) dissolving bismuth 2-ethylhexanoate in a first solvent to prepare a first solution; and b) dissolving strontium acetate in a second solvent to prepare a second solution.
And c) dissolving tantalum ethoxide in a third solvent to form a third solution.
D) mixing the solutions prepared in each of the steps a) to c) to prepare one homogeneous solution; and e) disposing the homogeneous solution on a substrate. Depositing to form a film on the substrate, wherein the steps a) to d) are performed at ambient room temperature. 2. The method according to claim 1, further comprising the step of annealing the substrate and the film at a temperature between 600 ° C. and 750 ° C. 3. The method according to claim 1, wherein said first solvent is 2-ethylhexanoic acid, said second solvent is acetic acid, and said third solvent is 2-methoxyethanol. . 4. The method of claim 3 further comprising the step of adding about 30% excess bismuth. 5. The method according to claim 1, further comprising the step of forming an upper electrode and a lower electrode, wherein the two electrodes sandwich the film to form a capacitor structure. 6. A step of preparing a first solution by dissolving bismuth 2-ethylhexanoate in a first solvent; and b) dissolving strontium acetate in a second solvent to prepare a second solution.
C) dissolving isopropyl titanate and tantalum ethoxide in a third solvent to prepare a third solution; and d) each of the steps a) to c). Mixing the prepared solutions to prepare one homogeneous solution; e) depositing the homogeneous solution on a substrate and depositing (1-
x) a step of forming a SrBi ₂ Ta ₂ O ₉ -xBi ₃ TiTaO ₉ film, wherein the steps a) to d) are performed at ambient room temperature. 7. The method of claim 6, further comprising the step of annealing the substrate and the film at a temperature between 600 ° C. and 750 ° C. 8. The method according to claim 6, wherein the first solvent is 2-ethylhexanoic acid, the second solvent is acetic acid, and the third solvent is 2-methoxyethanol. . 9. The method according to claim 6, further comprising the step of forming an upper electrode and a lower electrode, wherein the two electrodes sandwich the film to form a capacitor structure. 10. A step of preparing a first solution by dissolving bismuth 2-ethylhexanoate in a first solvent; and b) dissolving strontium acetate in a second solvent to prepare a second solution.
C) dissolving isopropyl titanate, tantalum ethoxide, and niobium ethoxide in a third solvent to prepare a third solution; and d) preparing the solution from a) to c). Mixing the solutions prepared in each of the steps to prepare one homogeneous solution; and e) depositing the homogeneous solution on a substrate and forming (1-
x) a step of forming a SrBi ₂ Ta ₂ O ₉ -xBi ₃ TiNbO ₉ film, wherein the steps a) to d) are performed at ambient room temperature. 11. The method of claim 10, further comprising the step of annealing the substrate and the film at a temperature between 600 ° C. and 750 ° C. 12. The method according to claim 10, wherein the first solvent is 2-ethylhexanoic acid, the second solvent is acetic acid, and the third solvent is 2-methoxyethanol. . 13. The forming method according to claim 10, further comprising a step of forming an upper electrode and a lower electrode, wherein the two electrodes sandwich the film to form a capacitor structure.